1.) Field of the Invention
This invention generally relates to a metal-made honeycomb body (hereinafter called the "metallic honeycomb body") useful as a principle component of an exhaust gas cleaning device for an automotive vehicle in order to support thereon an exhaust gas cleaning catalyst.
More specifically, this invention provides a metallic honeycomb body, a principal component of an exhaust gas cleaning device of the above-mentioned type employed under severe conditions, which has been improved in durability against deformation and breakage which would occur by expansion and stress under heat.
2.) Description of the Related Art
The construction of a typical conventional exhaust gas cleaning device A' of the above-described type is shown in FIG. 5 through FIG. 7.
Namely, the typical conventional exhaust gas cleaning device A' is constructed of a stacked multi-layer structure of a honeycomb structure, in other words, a metallic honeycomb body H' and a cylindrical metallic casing (hereinafter called the "metallic casing") 4. The metallic honeycomb body H' has been formed by superposing a planar metal band 1' of a heat-resistant steel sheet and a corrugated band 2', which was formed by corrugating a similar steel sheet, one over the other in a contiguous relation and then rolling them into a spiral form (see FIG. 5) or by stacking such planar bands and corrugated bands in layers (see FIG. 6), thereby defining a number of network-patterned gas flow passages (which may hereinafter be called "cells") along a central axis of the resulting metallic honeycomb body. The cylindrical metallic casing 4 is open at opposite ends thereof so that the metallic honeycomb body H' can be inserted and fixed in the casing.
Incidentally, FIG. 7 illustrates the structures of the planar and corrugated bands 1',2' making up the metallic honeycomb body H' of the rolled type (see FIG. 5) or the stacked (layered) type (see FIG. 6) as well as the positional relationship between these bands 1' and 2'.
In the present field of art, the above-described exhaust gas cleaning device A' is also called a metallic converter because it is constructed of the metallic honeycomb body and the metallic casing.
The planar band and the corrugated band, which make up the metallic honeycomb body, as well as the metallic honeycomb body and the metallic casing are firmly fixed together by a fixing method such as brazing or welding at points or areas of contact there-between, so that the resulting exhaust gas cleaning device A' can withstand thermal expansions and thermal stress, which occur because of the high temperature of exhaust gas itself and exothermic reactions of exhaust gas induced by a cleaning catalyst, and also vibrations during running of an associated automotive vehicle.
From the standpoint of a price competition with exhaust gas cleaning devices which are each equipped with a conventional honeycomb body made of a cordierite ceramic, there is a recent move toward constructing an exhaust gas cleaning device without using a separately-fabricated metallic casing for enclosing and firmly holding a metallic honeycomb body therein, in other words, by using only a metallic honeycomb body. As no metallic casing is used in this case, the fabrication cost of the metallic casing as well as the cost for inserting and fixing the metallic honeycomb body in the metallic casing, that is, the canning cost are no longer needed, thereby obviously leading to a substantial cost merit.
As will be explained below, the above-described conventional exhaust gas cleaning devices--which are each constructed of only metallic honeycomb body fabricated using a planar band and a corrugated band made of metal sheets--however cannot withstand long-term use even if they are provided with metal casings.
Based on thermal expansion and thermal stress which are produced in an atmosphere of the high temperature of exhaust gas itself and the heat generated through catalytic reactions of unburned gas induced by an exhaust gas cleaning catalyst, significant deforming force occurs especially in a direction perpendicular to an axial direction (i.e., the direction in which exhaust gas flows in and passes) of the metallic honeycomb body, to say nothing of the axial direction. The direction perpendicular to the axial direction will hereinafter be called the "radial direction of the metallic honeycomb body". Because of a temperature gradient between a central part of the metallic honeycomb body and its outer peripheral part, the deforming force tends to propagate close to the outer peripheral part of the metallic honeycomb body or close to points or areas of contact between an outer peripheral wall of the metallic honeycomb body and an inner peripheral wall of the metallic casing through the component members (the planar band and the corrugated band) of the metallic honeycomb body.
In the course of the propagation of the thermal deforming force, the component members of the metallic honeycomb body may be deformed, buckled, cracked or otherwise broken. In particular, the thermal deforming force concentrates near the outer peripheral wall of the metallic honeycomb body or around the points or areas of contact between the outer peripheral wall of the metallic honeycomb body and the inner peripheral wall of the metallic casing, the planar and/or corrugated bands made of the metal sheets and forming the metallic honeycomb body at such places also undergo substantial cracking, breakage or buckling, and separation or detachment takes place at points or areas of contact between the respective bands or at points or areas of contact between the metallic honeycomb body and the metallic casing. These problems significantly impair the durability of the exhaust gas cleaning device.
As measures for the improvement of the durability of the metallic honeycomb body especially in the radial direction under thermal deforming force, the following proposals have been made:
i) Japanese Patent Application Laid-Open (Kokai) No. SHO 63-182038 discloses a technique for fabricating a metallic honeycomb body from a first corrugated band and a second corrugated band having mutually-different wavelengths (periods) and waveheights (amplitudes). Namely, it is attempted to withstand radial pressure and expansion, which occur through thermal deformation cycles, by using a first corrugated band having a relatively large period and amplitude and a second corrugated band having a relatively small period and amplitude. PA0 ii) Japanese Patent Application Laid-Open (Kokai) No. SHO 64-30651 discloses a technique for fabricating a metallic honeycomb body from a planar band and a corrugated band. The planer band has curved concave surface portions formed in advance so that the planar band can be brought into fact-to-face contact with the curved convex surface portions (the ridges and grooves) of the corrugate band. Hence, the planer band may also be regarded as a kind of corrugated band. A primary object is to bring both the bands into face-to-face (inner/outer) contact at their respective convex/concave surface portions, thereby making it possible to improve the joining strength and also to save expensive .gamma.-alumina upon wash coating. Moreover, the planar band is in a wavy form as described above so that as a secondary advantage, radial thermal deforming force can be reduced by the curved surface portions. PA0 iii) Japanese Utility Model Application Laid-Open (Kokai) No. HEI 2-150032 discloses a metallic catalyst substrate of the structure that small-wave planar plates having a number of small waves (which may also be called "micro-corrugation") and flattened-top corrugated plates, in each of which top portions of waves have a flat shape, are alternately stacked. Each small-wave planar plate is so dimensioned that two or more small waves are brought into contact with each flattened top portion of the associated flattened-top corrugated plate. An object of this invention is to prevent filming-out (telescoping) of the metallic honeycomb body in the axial direction. This proposal however involves various problems such that the small-wave planar plates are difficult to fabricate because their small waves have a sinusoidal waveform with a small pitch (wavelength), the waveform may be stretched to its full length due to working stress applied upon fabrication of the metallic honeycomb body by rolling, and upon brazing or catalyst coating, the small wave portions may be clogged to result in a large back pressure. PA0 iv) Japanese Utility Model Application Laid-Open (Kokai) No. HEI 2-83033 discloses a technique for fabricating a metallic catalyst substrate by using only corrugated plates having a corrugated structure of such a modified waveform that openings of lower portions are formed narrow and upper portions are formed broad, for example, only modified corrugated plates having an .OMEGA. shape (in the form of the "omega" character) or an inverted triangular shape (in the form that triangles are arranged by alternately reversing their directions)
This technique however requires, as a premise, use of corrugated plates of sinusoidal curves or like curves as the first and second corrugated plates. Further, with respect to the manner of contact between the first corrugated plate and the second corrugated plate, it is difficult to take the structure that the corrugated plates are maintained in contact with each other in a ridge-to-groove or groove-to-ridge relationship (needless to say, this contacting relationship is preferred for the assurance of strength at the points or areas of contact). Even if both the corrugated bands are fixed together by brazing at points of contact therebetween, they are caused to separate from each other under the above-described large thermal deforming force so that various drawbacks will be developed in association with the separation, such as breakage of the corrugated plates.
In the above-described proposal, however, many of the curved convex surface portions formed on the planar plate are maintained in face-to-fact contact with the ridge portions (or the groove portions) of the corrugated plate, resulting in the drawback that the characteristic property of the planar plate to absorb and lessen the thermal deforming force is reduced.
The metallic honeycomb body which makes up the metallic catalyst substrate is however fabricated only from the deformed corrugated plates described above, so that it has a rigid structure and is hence insufficient in its absorbing and lessening effects for thermal deforming force.
As has been described above, a great deal of efforts has been made to develop a metallic honeycomb body which is suitable for use in an exhaust gas cleaning device and has a structure sufficient to withstand thermal deforming force. However, no fully satisfactory metallic honeycomb body has been obtained yet.